Preheat the oven to 350°F.

Make the syrup: In a small saucepan, bring the sugar and water to a boil until the sugar is completely dissolved. Add the rosewater and salt. Lower the heat and simmer until a slightly thicker syrupy texture forms, about 15 minutes. Remove from heat and let cool completely (you can stick it in the fridge if necessary to speed up the process).

While your syrup simmers, make the filling: place the walnuts, pistachios, sugar and cinnamon in the bowl of a food processor and pulse until coarsely ground (around 10 pulses). Set aside.

Unroll the phyllo dough. Place a 9x13” pan over the dough and measure and trim it to fit your pan. Discard the scraps and cover the dough with a damp cloth to keep it from drying out as you work.

Using a pastry brush, grease the bottom and sides of the pan with the melted butter.

Layer 12 sheets of the phyllo dough into the pan one at a time, brushing each sheet with butter before adding the next and keeping the remaining dough covered with a damp towel as you work.

Evenly sprinkle the nut mixture on top of the 12th sheet of phyllo.

Layer 12 more sheets of phyllo on top of the nut mixture, brushing each with butter before adding the next. Be sure to brush the final, top layer generously with the butter.

Using a sharp knife, begin to section the baklava. Cut four even 1 ½ inch rows vertically along the longer edge of the pan. Be sure to cut through all of the layers of phyllo dough to the bottom of the pan. Then, cut diagonally across the pan from corner to corner to form 1 inch wide diamond shapes.

Bake for 20 minutes on the bottom rack of the oven, then transfer to the top rack of the oven and bake for an additional 15-20 minutes until golden brown.

Remove from the oven and immediately spoon most of the cooled syrup over the hot baklava (listen for the crackle!). Let cool for at least 1 hour before serving, best if you wait overnight.

Store in an airtight container or freeze for up to 2 months.